1. Field of the Invention
This invention relates generally to apparatus and methods for producing heat exchange with body tissue, and more specifically to methods and apparatus for the initiation of hypothermic treatment of a body fluid in a body conduit or maintaining normothermia within the body conduit.
2. Discussion of the Prior Art
Many of the advantages of hypothermia, as well as maintaining normothermia, are well known. By way of example, it has been found particularly desirable to lower the temperature of body tissue in order to reduce the metabolism of the body. In stroke and several other pathological conditions, hypothermia also reduces the permeability of the blood/brain barrier. It inhibits release of damaging neurotransmitters and also inhibits calcium-mediated effects. Hypothermia inhibits brain edema and lowers intracranial pressure.
In the past, hypothermic treatment has been addressed systemically, meaning that the overall temperature of the entire body has been lowered to achieve the advantages noted above. This has been particularly desirable in surgical applications where the reduced metabolism has made it possible to more easily accommodate lengthy operative procedures. An example of this systemic approach includes catheters for transferring heat to or from blood flowing within a patient""s vessel, as disclosed by Ginsburg in U.S. Pat. No. 5,486,208. A closed loop heat exchange catheter is also disclosed by Saab in U.S. Pat. No. 5,624,392.
Some of the disadvantages of systemic hypothermia include cardiac arrhythmia, pulmonary edema and coagulopathies. Systemic hypothermia also results in hypotension and various immunodeficiencies.
The systemic approach is not always advantageous when the beneficial effects are desired locally at the focus of the operative procedure and only the disadvantages of hypothermia are felt throughout the remainder of the body.
More recent focus has been directed to producing hypothermia in localized areas of the body, leaving the remainder of the body to function at a normal body temperature. These localized applications of hypothermia have been external, relying for example on cooling helmets or cooling neck collars to produce localized hypothermia for the brain.
Another area of catheter construction involves heat pipe technology. Heat pipes are ideally packaged in thin metal envelopes (or tubes) due to the nature of good thermal conductivity and high pressure vessel strength. The heat pipe is ideal from the standpoint that no moving parts are required and no pumping mechanisms are needed to move the working fluid. An example of a heat pipe used as a catheter for precise temperature control for treating diseased tissue is disclosed by Fletcher in U.S. Pat. No. 5,591,162. However Fletcher does not teach a catheter with flexible bellows to allow for insertion into the human bloodstream. Nor does Fletcher disclose a catheter which can be used in the ambulatory setting.
In many of the neurological injury scenarios, time to treatment or the onset of hyperthermia is critical to the patient and ultimate recovery. It would be ideal if treatment could be initiated as soon as the patient presents or is reached. This can often be outside of a hospital setting. Therefore, a means to initiate hypothermia or maintain normothermia in an ambulatory environment would be greatly beneficial. The current state of technology, (i.e. recirculating catheters, water blankets, etc.) requires significant pieces of equipment to provide a cooling source along with tubing and heat exchangers to couple the cold source to the patient.
Current ambulatory methods of initiating hypothermia are ice baths, alcohol wipes, eliminating clothing and fanning. In hospital management, tools include water blankets and drug therapy. Such ambulatory methods listed are all topical or superficial cooling methods. Topical cooling is slow and labor intensive. In cases of brain injury, the patient does not necessarily have an elevated temperature at the time of injury. However, if the patient""s temperature can be managed from the time of insult, the better the chances are of maintaining normothermia or preventing severe elevated temperatures or fever spikes. Some applications such as heat exposure, burn victims, systemic infections, etc., may already be febrile and could benefit from immediate cooling. Systemic intravascular cooling is more effective than topical cooling since the body core temperature is directly effected by bloodstream temperature. Topical cooling does not always accomplish core cooling as desired. Therefore there has been a long felt need for an indwelling catheter which is portable for use in the ambulatory setting and flexible to navigate through the tortuosity of the blood vessels atraumatically.
A heat exchange catheter and method of operation are included in the present invention. The method is adapted to produce hypothermia, hyperthermia or maintain normothermia in a selected portion of the body without substantially varying the temperature of the remaining portions of the body. The selected body portion will typically be associated with a body conduit which conveys a body fluid to the selected body portion. Of particular interest are the organs of the body which are commonly nourished and maintained by a flow of blood in the arterial system. For example, a flow of blood is introduced to the brain through the carotid artery. Of course the temperature of this blood is usually at the normal body temperature.
By positioning a heat exchange catheter in the body conduit, heat can be added to or removed from the body fluid to heat or cool the selected body portion. For example, the heat exchange catheter can be disposed in the carotid artery where the arterial blood flowing to the brain can be cooled. The flow of cooled blood to the brain reduces the temperature of the brain thereby resulting in cerebral hypothermia. Importantly, this temperature reduction occurs primarily and selectively in the brain; the remaining portions of the body maintain a generally normal body temperature. In accordance with this method, the selected body portion, such as the brain, can be cooled thereby providing the advantages associated with hypothermia for this body portion. The remainder of the body, such as the portions other than the brain, do not experience the reduction in temperature and therefore are not susceptible to the disadvantages of hypothermia. Furthermore, the invention is intended to remotely alter temperature in a region other than the point of introduction into the body. This is different than devices intended for systemic temperature control.
Several factors are of interest in effecting heat transfer in a heat exchanger. These factors include, for example, the convection heat transfer coefficient of the two fluids involved in the heat exchange, as well as the thermal conductivity and thickness of the barrier between the two fluids. Other factors include the relative temperature differential between the fluids, as well as the contact area and residence time of heat transfer. The Reynolds number for each fluid stream affects boundary layers, turbulence and laminar flow.
With concern for these factors, the heat exchange catheter of the present invention includes a shaft having an axis, a fluid inlet lumen and a fluid outlet lumen each extending generally between a proximal end and a distal end of the shaft. A hub disposed at the proximal end provides access to the fluid lumens. At least one balloon is provided in a heat exchange region at the distal end of the shaft, the balloon wall providing the barrier between the two fluids. With the catheter positioned in contact with the body fluid within the conduit, heat transfer occurs across the balloon wall. The relative temperature differential is facilitated with countercurrent flow between the two fluids.
In one aspect of the invention, a first balloon is disposed at the distal end of the shaft and defines with the shaft an inflatable first cavity. Portions of the shaft define a first inlet hole extending in fluid communication between the first lumen and the first cavity. Portions of the shaft define a first outlet hole extending in fluid communication between the first cavity and the fluid outlet lumen. A second balloon disposed relative to the first balloon defines with the shaft an inflatable second cavity with portions of the shaft defining a second inlet hole between the fluid inlet lumen and the second cavity. Portions of the shaft also define a second outlet hole in fluid communication with the second cavity and the fluid outlet lumen. Typically, the first balloon will be disposed distally of the second balloon and the first inlet hole will be larger than the second inlet hole. An elastomeric material covering a valley or volume between the first balloon and the second balloon may be provided to promote mixing necessary for efficient heat exchange yet minimize turbulence and shear which can be damaging to blood.
In an additional aspect of the invention, a method for exchanging heat with a body fluid in a body conduit includes the step of introducing into the body conduit a catheter having an inlet lumen and an outlet lumen. The catheter is provided with a first cavity and a second cavity each in heat transfer relationship with the body fluid in the body conduit. A heat exchange fluid is introduced into the inlet lumen and through an inlet hole into each of the first cavity and the second cavity. An exchange of heat then occurs between the heat exchange fluid in the first and second cavities and the body fluid in the body conduit. Ultimately, the heat exchange fluid is removed through an outlet hole and the outlet lumen associated with each of the first cavity and the second cavity. Creating non laminar flow in one or both of the heat exchange fluid and the body fluid will improve heat transfer efficiency. Heat transfer can also be erected by various structures which either enhance or inhibit turbulence in the fluids.
The present invention addresses the shortcomings of the prior art by allowing for heat pipe technology to be applied to intravascular catheters. The heat pipe allows for a heat exchange catheter design that does not require the circulation of a heat exchange coolant to and from the catheter. Furthermore, the invention provides for portability and use in situations where cumbersome equipment would normally preclude treatment. It also provides for a catheter design that could be used in two phases: ambulatory with a portable cooling source and then bedside with a stationary cooling source. All of this is accomplished without having to exchange catheters in the patient. An additional aspect of the invention includes metallic bellows to provide for the desired flexibility while maintaining the benefits of heat pipe technology.
The heat pipe heat exchange catheter of the invention is comprised of a tubular metal envelope that is hollow and hermetically sealed. Inside this hollow tube is a conventional heat pipe construction containing a capillary element and a heat exchange fluid such as water. In one embodiment, the heat exchange fluid comprises a fluid and gas vapor which are homogeneous. The construction, sizing and assembly techniques of the heat pipe permits the flow of energy from one portion of the pipe to the other. In an application in accordance with the invention, the indwelling portion of the catheter lies in the bloodstream at approximately 37xc2x0 C. One embodiment discloses the indwelling portion of the catheter to be placed into the portion of the human body by using a sheath. In yet another embodiment, the heat pipe heat exchange catheter is inserted into the body by using a guidewire which is inserted through an inner hollow shaft of the heat pipe heat exchange catheter. This inner shaft may be the wick of the heat pipe.
The external portion of the catheter is placed in thermal contact with a cooling mechanism so that there is a temperature differential between the different portions of the catheter. The heat pipe fluid at the warmer blood portion boils and vaporizes. The vaporization phase change draws heat energy from the blood thus cooling the blood. The hot vapor then travels down the tube to the cold portion (at approximately 0xc2x0 C.) where it condenses into liquid again. This exothermic phase change occurs because this portion of the catheter is being chilled by an external source. The now condensed liquid then travels down a capillary wick to the blood portion of the catheter to start the cycle over again. As long as there is a temperature differential, the energy transfer will continue.
In order for a catheter to be metal and contain the elements of a heat pipe, an aspect of the construction of the instant invention utilizes the process of metal deposition used to form a heat pipe catheter body. This process utilizes a sacrificial mandrel to be coated with a metal alloy. Once the wall thickness of metal is sufficient, the mandrel is dissolved and a hollow tube is left. The shape of the mandrel determines the shape of the final metal body. The mandrel can be shaped in any form including bellows which in turn would allow the catheter to have flexibility or flexible joint sections. This flexibility is necessary to navigate the tortuosity of the blood vessels atraumatically. In one embodiment, a biocompatible metallic plating covers the base metal to assure biocompatibility with the human body; examples of such plating are gold, nickel cobalt alloy, stainless steel, pyrolitic carbon and anodized aluminum.
Another aspect of the invention discloses a coating applied to the body of the heat pipe heat exchange catheter. By way of example, the coatings include, but not limited to, the following: a thromboresistant coating (such as heparin or albumin), a clot lysis coating (such as enzymes), an antimicrobial coating (such as antibiotics or silver Ag), and/or a lubricious coating (such as hydrogel).
The second portion of the invention provides the cooling capacity which is the driving force for energy transfer of the heat pipe. The heat pipe catheter design described above lends itself to be coupled to many types of cooling sources. Its high efficiency could justify jacketing the heat sink portion with conventional liquid cooling systems such as COOLGARD(trademark) (U.S. patent app. Ser. No. 09/220,897, filed on Dec. 24, 1998, entitled xe2x80x9cCooling System for Indwelling Heat Exchange Catheterxe2x80x9d) available from Alsius Corporation in Irvine, Calif. However, its operational characteristics make it unique in that xe2x80x9ccoldxe2x80x9d applied to the heat sink portion makes it work.
As disclosed in Alsius U.S. Pat. Ser. No. 6,019,783 (U.S. patent App. Ser. No. 09/260,950 filed Mar. 2, 1999), the first proposed cold source is a compact cooler based on thermoelectric cooler technology. These solid state devices are small and run off DC current (available in ambulances, portable batteries, etc.).
A second cold source is based on bottled liquid/gas. In this aspect, the heat sink portion of the catheter is jacketed (enveloped in a manifold) and is coupled to a canister of bottled gas. Preferably, this gas is inert and not harmful, such as CO2. If this CO2 is bottled in a liquid form, it can be released in a metered fashion in the form of a very cold gas vapor. This cold gas being a product of the phase change process. A metering device could be used as a throttle to control the temperature of the heat sink portion of the catheter. The compactness of the bottle and the gas being coupled to the catheter via a hose eliminates having a heavy mechanism attached directly to the catheter. Thus the cannister can be controlled with a control valve and a metered vaporization orifice coupled to at least one hose.
Accordingly, in one aspect of the present invention, an apparatus for effecting heat exchange with at least a portion of a human body is disclosed. The apparatus comprises of a heat pipe heat exchange catheter and a portable temperature control module. The heat pipe heat exchange catheter is configured for insertion into a portion of the human body. The catheter can be inserted a number of ways, two of the preferred methods being that of either using a guidewire through the hollow shaft of the heat pipe or using a sheath. The portable temperature control module provides the driving force for the energy transfer between the heat pipe heat exchange catheter and the portion of the human body.
In yet another preferred embodiment, a temperature control device having a plurality of articulated portions, at least one portion being an implantable heat pipe heat exchange catheter, and a temperature control module. The implantable heat pipe heat exchange catheter comprises of a hermetically sealed metal envelope. The driving force for the energy transfer between the implantable heat pipe heat exchange catheter and the human body is provided by the temperature control module.
In accordance with the invention there is provided a new and improved method for effecting heat exchange with at least a portion of a body of a patient. The method including inserting a flexible heat pipe heat exchange catheter into a blood vessel of the patient, wherein the heat pipe heat exchange catheter includes an indwelling heat exchange region and an ex vivo heat load dissipater or collector region, and thermally coupling a temperature control module to the ex vivo heat load dissipater or collector region. The method thereby drives a heat exchange interaction between the indwelling heat exchange region and the portion of the body.
Optionally, the temperature control module may be portable. In an embodiment the insertion includes using a sheath. In yet another embodiment, the insertion includes using a guidewire through the inner hollow shaft of the heat pipe. This inner hollow shaft may be the wick of the heat pipe.
For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appending claims.